1) Field of Invention
This invention relates to compression-evaporation cooling equipment.
2) Description of Related Art
Heretofore, compression-evaporation cooling systems have relied on mechanical compressors for their operation. Although compression-evaporation systems offer comparatively high efficiency, the use of mechanical compressors requires certain design comprises, which serve to reduce the refrigeration system's overall efficiency.
It is well known, that "flashing" of the liquid refrigerant as it enters the evaporator, reduces the refrigerating effect per pound of refrigerant. This sudden liquid-to-gas change of state, occurs when the liquid refrigerant cools from the condensing temperature to the evaporator temperature. Since this change of state comes at the expense of the liquid refrigerant's internal energy, no useful cooling occurs. Flashing can be reduced by subcooling the liquid refrigerant before it enters the evaporator. However, significant subcooling requires another cooling system with its own energy requirements. To determine the benefit of subcooling in a given system, the energy saved due to reduced flashing, must be compared with the energy consumed by the subcooling system.
Heat exchangers between the suction vapor and the liquid refrigerant have been employed in smaller systems to provide subcooling. However, the loss of efficiency associated with suction vapor superheating, can limit the efficiency gain of this kind of subcooling.
Mechanical compressors which are employed in compression-evaporation systems provide a fixed displacement, which is difficult to vary during operation. Thus, their discharge pressure is also difficult to vary. For compression-evaporation systems, the compressor's discharge pressure must be high enough to provide condensation at the highest temperature of the condensing medium. As such, the design choice of the compressor's discharge pressure must be made on a worst-case basis. During periods when the condensing medium's temperature is below this worst-case temperature, the discharge pressure of the compressor is larger than the minimum pressure required for condensation to occur. Therefore, during normal operating conditions, energy is wasted by producing excessive discharge pressures.
For example, a compressor's discharge pressure for a typical residential refrigerator might be designed to sustain condensation at room air temperatures of up to 100 degrees Fahrenheit. During periods when the room's air temperature is below 100 degrees Fahrenheit, a lower discharge pressure could sustain condensation. Thus, during periods of average room air temperatures, the compressor wastes energy by producing discharge pressures which are higher than necessary. Also, the selection of electric motors is made on this same worst case basis. The electric motor must be capable of startup and pulldown of a warm refrigerator, during periods of high room temperature. Consequently, a motor must be used whose power consumption is greater than the minimum required for normal operation.
In short, any compression-evaporation system where the condensing medium's temperature changes, will suffer from these inefficiencies. These fixed discharge pressure considerations can also be applied to heatpumps and air-conditioners. During periods when the indoor-outdoor temperature difference is small, the minimum pressure differential needed is reduced. Since mechanical compressors cannot easily vary their displacement, compression-evaporation systems are unable to exploit the increased efficiency of a variable discharge pressure.
The design of mechanical compressors with variable displacement, has always led to the addition of many more moving parts. These extra moving parts decrease the compressor's efficiency and dependability. Consequently, the advantages offered by a variable discharge capacity compressor can provide gains in overall system efficiency. Variable capacity compressors have been achieved in the past, by combining variable speed electric motors with mechanical compressors. However, such systems have never offered both variable capacity and variable discharge pressure in a single compressor.
It is clear that there is a need for a compressor technology which can provide an efficient subcooling system, a variable discharge pressure, and variable capacity. If such a compressor technology were available, the efficiency of compression-evaporation cooling systems could be advanced considerably.